Janko Tipsarevic, of Serbia, returns a shot to David Ferrer, of Spain, during the fourth round, Sept. 2, 2013.

Credit: David Goldman/AP

Mike Bryan, right, and his twin brother Bob Bryan check their racquets during their match against Colin Fleming and Jonathan Marray, both of Britain, during the men's quarterfinal round, Sept. 2, 2013.

Credit: Charles Krupa/AP

Tommy Robredo, of Spain, chases down a shot against Roger Federer, of Switzerland, during the fourth round, Sept. 2, 2013.

Credit: Darron Cummings/AP

Sloane Stephens returns a shot to Serena Williams during the fourth round of the 2013 U.S. Open tennis tournament, Sept. 1, 2013.

Credit: David Goldman/AP

Sloane Stephens returns a shot to Serena Williams during the fourth round of the 2013 U.S. Open tennis tournament, Sept. 1, 2013.

Credit: Darron Cummings/AP

Marcos Baghdatis, of Cyprus, reacts during a third round match against Stanislas Wawrinka, of Switzerland, Sept. 1, 2013.

Credit: Mike Groll/AP

Lleyton Hewitt, of Australia, serves against Evgeny Donskoy, of Russia, during the third round, Sept. 1, 2013.

Credit: Kathy Willens/AP

Angelique Kerber, of Germany, returns a shot to Carla Suarez Navarro, of Spain, during the fourth round, Sept. 1, 2013.

Credit: Mike Groll/AP

Andy Murray, of Great Britain, serves to Florian Mayer, of Germany, during the third round, Sept. 1, 2013.

Credit: David Goldman/AP

Angelique Kerber, of Germany, returns a shot against Carla Suarez Navarro, of Spain, during the fourth round, Sept. 1, 2013.